In the past, "wet wrap" or "torsion box" skis have typically been formed by impregnating a sheet of unidirectional fiberglass with epoxy resin. The core and any other internal components of the ski, such as fiberglass mat or a bias-ply precured fiberglass strip, are placed in the center of the unidirectional glass, again with resin applied. The unidirectional glass is then wrapped tightly around the core on all four sides. This unit is then placed in a mold, the base assembly set on top, and the mold closed. The unit is "cooked" under fairly high pressure for a period of time and, after the cure cycle, is removed from the mold. At this point, the ski is basically complete structurally and the rest of the production process is devoted to sanding, topping, finishing and other cosmetic operations. Ski cores have typically been wrapped or "laid up" by hand and this is a labor intensive and time-consuming process.
It is known that unidirectional fiberglass is a material of great longitudinal tensile strength but little lateral strength. Thus, the torsional rigidity of skis was typically increased through the addition of randomly oriented fiberglass mat or angularly biased, precured fiberglass strips to the "sock" unit prior to wrapping. The longitudinally oriented unidirectional fiberglass imparted longitudinal rigidity to the ski. It is known that the orientation of fiberglass at a 45 degree angle to the longitudinal dimension of the ski core imparts the highest torsional rigidity to the ski. Of necessity, the use of a randomly oriented mat to increase torsional rigidity also resulted in unnecessarily increasing ski weight and expense since only a small percentage of the mat fibers were oriented at angles which enhanced torsional stiffness.
The placement of 45 degree biased, precured fiberglass strips in the "sock" was an improvement in that it represented a more effective use of the strength characteristics of fiberglass in controlling torsional rigidity, but was undesirable in that it required additional lay-up and curing steps for the strips themselves. Further, the biased fiberglass was not wrapped around the core and thus did not obtain the benefits of strength and durability provided thereby.
Another problem of the prior construction technique was that variations in lay-up from ski-to-ski created differences in torsional and longitudinal rigidity, thus making it difficult to produce a truly matched pair of skis. Further the known wrap process involves wrapping a rectangular sheet of fiberglass around the ski core which has a varying cross section along its length. This creates a large overlap of the sheet at the narrow waist of the ski and a small overlap at the wide tip. This adversely affects molded tolerances in the ski.
The problems of hand lay-up have been attempted to be overcome in the past through the use of prebraided bias fiberglass socks which were slipped over a ski core prior to resin impregnation. Such socks were formed of multiple strands of fiberglass oriented at angles with respect to each other such that when the finished sock was slipped over the ski core, the strands were also oriented at angles, preferably 45 degree angles, to the ski core. Such prebraided fiberglass socks were difficult to use in that the braid tended to become loosened and unbraided while being slipped onto the ski core. In addition, the sock invariably fit loosely upon the ski core, thus creating difficulties in manufacture as well as quality control. The technique was also labor intensive and inflexible in design and still required premade contoured strips of longitudinal fiberglass.
Prebraided fiberglass socks including longitudinally extending strands of fiberglass, as well as the angularly biased strands, were successful in overcoming the tendency of the solely angularly biased strand prebraided socks to become unbraided. Such three strand socks, however, had such little ability to expand that it was extremely difficult to slip them onto a ski core and they could not be effectively used in a production process.
The present invention provides a unique ski core covering whereby strands of fiberglass are braided at preselected angles directly onto the core as the core is moved through a braiding machine. In addition, longitudinally directed strands of fiberglass may also be intertwined with the angularly biased strands during braiding. Other strands of longitudinally extending fiberglass or other desired materials may be positioned on the ski core while it is being passed through the braiding machine to allow for additional strengthing and tailoring of the flex characteristics of the ski.
The present invention makes possible a lighter and yet stronger ski in that the use of randomly oriented fiberglass is eliminated and all strands which are used are oriented longitudinally such that the best use can be made of their tensile strength characteristics.
The amount of fiberglass braided onto the ski core, as well as the angle of the braided fiberglass strands with respect to the ski core, can be varied by increasing or decreasing the speed of movement of the ski core through the braiding machine or the speed of the braiding machine itself. The equipment may be operated to allow the angle of the braided fiberglass strands with respect to the longitudinal dimension of the ski core to be varied along the ski core to separately control the torsional rigidity characteristics of the shovel or tail of a ski as desired. The braiding of the fiberglass onto the ski core under slight tension prevents unbraiding and holds the orientation of the fiberglass with respect to the ski after it is removed from the braiding machine and prior to its impregnation with epoxy and placement into a mold.